THE ORIGIN OF THE LATERAL OF VERTE- 

 BRATES EYES 



PROFESSOR G. H. PARKER 

 Harvard University 



The steps by which the vertebrate sense organs have 

 arisen have been traced in some measure through the evi- 

 dence afforded by their embryonic growth. In this re- 

 spect few systems of organs are more homogeneous. It 

 seems to be a well established principle of embryology 

 that all the sense organs of a developing animal shall arise 

 from its outermost layer of cells, the ectoderm, and the 

 fact that sense organs are the chief means by which an 

 animal tests its environment shows that the embryonic 

 source of these organs is not without significance, for 

 what situation is more important for organs adapted to 

 the reception of environmental changes than the outer- 

 most surface of the animal that possesses them. These 

 conditions are well illustrated by most of the sense or- 

 gans possessed by the vertebrates. Our organs of touch 

 have for the most part remained where they were formed 

 in the outermost layer of the skin. The internal ear, 

 which in the great majority of vertebrates is a compli- 

 cated sac without connections with the skin, is known to 

 arise in individual development as a pocket from the skin 

 secondarily cut off from that layer. Without doubt it is 

 an excessively delicate organ of touch, for hearing and 

 the other sensory activities of the ear seem to be depend- 

 ent upon the same form of stimulus as touch, except 

 that the kind of stimulus appropriate to the ear is much 

 more refined than that for simple touch. Like the ear, 

 the vertebrate organs of taste and of smell are also prob- 

 ably derived from the skin and in their cases they still 

 retain the primitive connections with the mother layer, 

 for though they are in cavities, these cavities are lined 

 with tissue directly continuous at the mouth of the cavity 

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